A control system for controlling operation of an internal combustion engine detects a selected one of two exhaust modes: a first exhaust mode in which exhaust gases are guided through a first catalytic converter (16) upstream of a second catalytic converter (17), and a second exhaust mode in which exhaust gases are guided through a bypass passage (14a) bypassing the first catalytic converter (16). A control mode of the control system is determined according to which of the two exhaust modes is selected. The amount of fuel supplied to the engine and/or the ignition timing may be varied between values suitable for each exhaust mode. Correction of an air-fuel ratio control parameter based on the output from an oxygen sensor (32) may be inhibited when the second exhaust mode is selected.